The invention relates to a luminescent screen provided with a luminescent layer which is formed on a carrier and comprises a luminescent silicate having an apatite crystal structure. The invention further relates to a low-pressure mercury vapour discharge lamp provided with such a luminescent screen.
The Netherlands Patent Application No. 8006223 laid open to public inspection discloses luminescent screens comprising luminescent silicates of an alkaline earth metal and of a rare earth metal, these silicates having the hexagonal apatite crystal structure and corresponding to the general formula Me.sub.a Ln.sub.b (AO.sub.4).sub.6 X.sub.2. In this formula 9.ltoreq.a+b.ltoreq.10 and Ln represents gadolinium and, as the case may be, yttrium and/or lanthanum. Me is an alkaline earth metal, such as Ca, Sr, Ba, Mg and Zn, while A represents silicon, phosphorus and/or boron. X represents halogen, oxygen and, as the case may be, vacant sites. These known luminescent silicates are activated by lead or by lead and terbium and/or manganese. Such luminescent silicates, which, however, do not contain gadolinium and are activated by antimony, lead, tin, antimony and manganese or lead and manganese, are also described in the Netherlands published Patent Application 7005708, corresponding to U.S. Application Ser. No. 345,444, filed Mar. 27, 1973, now abandoned.
The element terbium is a frequently used activator for luminescent materials because it gives rise in many crystal lattices to a very efficient luminescence, in which the characteristic green Tb.sup.3+ emission is produced. Upon excitatinn by ultraviolet radiation, however, it is required for obtaining an efficient luminescence that the luminescent material is excited by radiation having a wavelength lying at the maximum or very close to the maximum of the excitation spectrum of the material.
An important application of such materials is found in low-pressure mercury vapour discharge lamps. In such lamps, mainly ultraviolet radiation having a wavelength of about 254 nm is produced. A great disadvantage of many Tb-activated materials is that the maximum of the excitation band of the terbium is found at wavelengths lying comparatively far from 254 nm. In those cases, an efficient luminescence can be obtained only if the excitation energy is first absorbed in a second activator, after which this energy is transferred to the terbium. During this absorption and transfer, of course losses may occur.
The invention has for its object to provide luminescent screens comprising new luminescent materials and more particularly materials which are activated by terbium and which can be directly excited in the terbium.